1. Field of the Invention
The present invention generally relates to a radio apparatus having multiple separate antennas, and in particular to a radio apparatus suitable for a portable transceiver or a portable receiver.
2. Description of the Related Art
In a radio apparatus having a pair of separate antennas providing spatial diversity, especially in a portable radio apparatus, it is difficult to ensure a sufficient spacing between the separate antennas, respectively. This causes isolation between antenna devices to be reduced, resulting in deterioration of antenna gain.
To overcome such a problem, a radio apparatus capable of selectively terminating the diversity antennas has been proposed in Japanese Patent Unexamined publication No. 7-202774. This conventional radio apparatus is provided with an antenna terminating system including a monopole antenna and a small size antenna which are connected to first and second antenna terminators through first and second switch devices, respectively. When the received signal level of the monopole antenna is higher than that of the small size antenna, the monopole antenna is connected to a receiver by the first switch device and the small size antenna is connected to the second antenna terminator by the second switch device. Otherwise, the monopole antenna is connected to the first antenna terminator by the first switch device and the small size antenna is connected to the receiver by the second switch device.
In the conventional radio apparatus mentioned above, a radio-frequency (RF) amplifier is not provided at the stage preceding the receiver. Thereforet a radio signal received by the receiver is reduced in signal level by feeder lines and the switch device, resulting in deterioration of the accuracy of received level detection. Further, the first and second switch devices must use a single-pole double-throw switch for selecting one of the paths to the receiver and the antenna terminator, resulting in difficulty in meeting the needs of both effective isolation and miniaturization.
An object of the present invention is to provide a radio apparatus which is capable of receiving radio signals through multiple antennas while meeting the needs of both miniaturization and improved isolation between multiple antennas.
Another object of the present invention is to provide a radio apparatus which is capable of ensuring the improved isolation and a sufficient level of received radio signal received through multiple antennas.
According to the present invention, each of a plurality of receiving systems is provided with an antenna terminator and a path control switch. The antenna terminator selectively terminates a corresponding antenna of the multiple antennas and the path control switch selectively switches on and off to provide a path for a radio signal received by the corresponding antenna. The antenna terminator and the path control switch are controlled such that an antenna terminator is inactive and a path control switch is on in a selected receiving system and, contrarily, an antenna terminator is active and a path control switch is off in the other receiving system.
Since an antenna terminator is inactive and a path control switch is on in a selected receiving system and an antenna terminator is active and a path control switch is off in the other receiving system, the isolation between multiple antennas is improved, promoting miniaturization.
The path control switch may be a preamplifier which is selectively on and off. When the preamplifier is on, the path is provided. When the preamplifier is off, the path is blocked off. More specifically, each receiving system includes an antenna terminator and a preamplifier. The preamplifier amplifies a radio signal received by the corresponding antenna, the preamplifier being selectively active and inactive. Further, the radio apparatus is comprised of a discriminator for discriminating one receiving system from another receiving system based on radio signals received by the receiving systems, respectively, and a controller for controlling the one receiving system such that an antenna terminator is inactive and a preamplifier is active to produce an amplified radio signal to be used as a received signal and controlling the other receiving system such that an antenna terminator is active and a preamplifier is inactive.